brcmfmac: introduce asynchronous firmware loading

The driver needs firmware to be loaded to the device, which
is done through the firmware class API. The synchronous call
request_firmware() need root filesystem to be mounted and/or
user-mode helper. These may not be avaliable on the moment
it is called. Instead use request_firmware_nowait().

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>